1. Field of the Invention
The present invention relates to a method for making reduced iron using blast-furnace sludge.
2. Description of the Related Art
Blast-furnace dust can be classified into relatively coarse dry dust collected by a dry system and sludge consisting of fine particles collected by a wet system. Dry dust has a relatively low zinc content when compared with that of sludge, and can be recycled as the material for sintered ore. In contrast, sludge having a high zinc content requires zinc-removing processes before being recycled as a blast-furnace feed material.
Recently, various methods for processing dust have been investigated. As a result of the investigation, a rotary hearth furnace method including the steps of agglomerating a mixture of an iron-oxide-containing substance, such as iron ore or dust, and a carbonaceous reductant and heating the resulting agglomerates in a rotary hearth furnace so as to obtain reduced iron has been put to practical application. According to this method, dust containing iron oxides can be reduced and can be effectively recycled as an iron source while achieving high production efficiency and low equipment cost.
In a rotary hearth furnace method, green pellets containing water are dried before they are fed to a rotary hearth furnace. The drying is necessary to prevent the pellets from bursting due to sudden evaporation during heating, i.e., to prevent “bursting” inside the furnace.
According to a known technique, when pellets made from a mixture of iron ore as the iron-oxide material and a carbonaceous material such as coal are used, the pellets are dried to reduce the water content thereof to 1 percent by mass or less. With this technique, bursting of the pellets inside the furnace can be prevented and handling strength, i.e., shattering strength, crushing strength, or the like, that can sufficiently withstand the feeding operation described above can be obtained.
However, when pellets contain blast-furnace sludge, the pellets break into chips and fines when they are fed inside the rotary hearth furnace even though they are dried to a water content of 1 percent by mass or less. As a result, a significant degree of pulverization occurs, and handling of the resulting reduced iron product becomes extremely difficult. This results in a decrease in metallization degree of the resulting reduced iron product, a decrease in zinc-removal rate, and a decrease in yield. Furthermore, a large amount of iron oxide becomes mixed in the recovered crude zinc oxide, thereby degrading the quality as the zinc oxide material.
Another known method for making reduced iron includes the steps of dehydrating a mixture containing undried blast-furnace sludge and so on by squeezing, extruding the resulting dehydrated mixture to form compacts, and feeding the compacts into a rotary hearth furnace without drying the compacts so as to perform drying, heating, and reducing of the compacts. According to this method, the degree of dehydration and the degree of powder filling are adjusted to prevent the compacts from bursting inside the furnace. However, since blast-furnace sludge expands during reduction as described below, breaking of the compacts cannot be effectively prevented by merely avoiding bursting resulting from water evaporation. Moreover, since the compacts are dried on the rotary hearth before reduction, the compacts remain in the furnace for a long time, thereby requiring a large hearth and a large-scale reduction plant.